Made parameters a struct in MATLAB export

This way both parameters and observables are structs, rather than having
parameters be instance fields and observables be contained within a
struct.
Modified get_observables function to return all observables

Observables are returned as a struct which can be indexed by name, e.g.

m = model()
[t y] = ode15s(@m.odes, [0 100], m.get_initial_values())
y_obs = m.get_observables(y)
y_obs.obs_name